This invention relates to electric vehicles and more particularly to field control features for use on an electric vehicle driven by a separately excited field control direct current drive motor.
In most prior art electric vehicle speed control systems, a direct current (DC) chopper control and a series wound drive motor are used. The chopper is placed in series between a DC power supply and a series wound drive motor. The DC chopper controls the speed of the DC series motor by controlling and interrupting the full armature current. The speed of the DC drive motor is controlled by conventional armature chopper apparatus which provides complete control of the motor over its entire speed range.
A prior art DC chopper controls the time of the pulses and/or the magnitude of the pulses being sent through the armature of the direct current drive motor. By varying the parameters of the drive pulses being fed to the armature of the DC drive motor the speed of the drive motor can be varied. DC chopper control for armature current coupled with a series wound drive motor is the most common approach used for driving prior art electric vehicles. Since a chopper controls the full armature current, it must of necessity be of a large size to handle the large currents involved. Prior art direct current choppers which are required in the armature circuit are bulky and expensive. The conventional chopper controller, since it must interrupt and control full armature current, requires expensive high-powered thyristor and complicates control circuitry for switching and controlling the load current in the armature circuit. Due to the large currents being interrupted, cooling of the controller is a problem. Conventional chopper controllers for armature current are also rather inefficient at low to normal operating speed.
An electronic system for controlling the torque speed characteristic and regenerative braking of a separately excited drive motor for use on an electric vehicle is disclosed in cross-reference U.S. patent application No. 346,552, Westinghouse Case No. 44,492. The torque speed characteristics can be controlled by varying the separately excited shunt field of the direct current drive motor. As disclosed in the above-mentioned applications, speed of the direct current drive motor is controlled from some base speed to an maximum speed. Field control of the separately excited DC drive motor has a number of advantages including lower cost and lighter weight than the conventional chopper control. A problem with the armature controller as there disclosed is that the speed range above the base speed is not wide enough. Furthermore, due to the dependence of field excitation on battery voltage the maximum motor speed varies excessively as a function of battery voltage, battery resistance and load. It is desirable to have a system that will operate smoothly over a wide speed range and will perform well in spite of changes in battery voltage, battery resistance and load. Maximum motor speed should be well controlled under extreme conditions including high throttle settings while descending a grade. An efficient low power dissipation system is desirable.